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Author Topic: Audio hiss on TS 870 - headphones jack  (Read 1072 times)
KQ7W
Member

Posts: 13




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« on: March 30, 2005, 06:16:24 PM »

Hello, I just purchased a
Heil proset headset, I
notice a hiss on my
TS870, even when the AF
knob is turned all the way
down, I never had this
issue with the Kenwood
HS5 headsets, I would
like to know what kind of
resistor do I have to
install inline the headset/
rig in order to get rid of
the hiss and match the
rig's impedance as I dont
have the specs on either
the rig or the headset, if
anyone noticed and fixed
this before please let me
know at mats4d at
yahooo . com
Thanks ! much
appreciated
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SWL1
Member

Posts: 29




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« Reply #1 on: March 30, 2005, 11:38:19 PM »

Hi there,

the size of the resistor depends on the impedence of your head phones.

Are they conventional low impedence around 8-Ohms or,the higher ones around 2-->3 K ?

What you might want to do is pick up a variable resistor /pot at a near by electronics store and,install in series with the hot lead fron the rig.

Turn the volume control on the rig up around 20 to 30% approximatley and,then adjust the pot for a quiter signal.

You may have you 'jockey' the settings between the rig level and,the in line att.

Once you are satisfied remove the pot and,measure it with an ohm meter.

Replace the pot with a fixed resistor and,your done Smiley

If the head phones are around 2 K ohms use a 10K ohm pot.

If they are low impedence type I would try around a 100 ohm pot.

It's not real critical however,if you don't use a high enough value you may not get enough attenuation?

Good luck.

73,

DE:Bob.
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N3ZKP
Member

Posts: 2008




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« Reply #2 on: March 31, 2005, 05:57:01 AM »

Matching impedences won't get rid of the hiss. That's the internal noise generated by the rig. You probably didn't hear it before because the Kenwood 'phones had a lower sensitivity than your new Heil.

Installing a resister in the line will only reduce the output to the phones. It won't actually get rid of the hiss, it will just knock down the volume level so you don't hear it as much.

You might try the phones conncted to the speaker output, instead of the phones jack and see what happens.

Lon
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KQ7W
Member

Posts: 13




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« Reply #3 on: March 31, 2005, 12:07:00 PM »

Thanks for the replies, much appreciated !

I will check both answers and see what I can find, the hiss is very minor, but once my brains "locks' onto it, it becomes very noticeable, it adds to the S7 power line noise I have Sad

Thanks again - hope these answers can help other people too.

Matt - KQ7W
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K0RFD
Member

Posts: 1368




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« Reply #4 on: March 31, 2005, 12:07:35 PM »

paralleling the phones with a capacitor will reduce the high frequency response (by shunting the highs to ground) and therefore some of the hiss.

The value of the capacitor you'd use depends on the impedance of the phones.

There are lots of programs on the internet for crossover network design, find one and plug some numbers into it.

You might not like the results, though.
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SWL1
Member

Posts: 29




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« Reply #5 on: March 31, 2005, 01:00:05 PM »

Hi again,

with all due respect to the other gentleman I have to disagree with him.

If you 'Pad down' the signal from the rig to the headphones and,turn up the volume control on the rig you effectively improve the signal to noise ratio.

Most of the hiss / noise on newer rigs is usually generated in the audio power amp section.

ie:After the volume control section.

A lot of companies use a cheap IC audio power amp like the TDA series which doesn't have the greatest signal to noise ratio.

By attenuating the signal from the rig to the headphones it enabels you to'drive' the headphones with a higher audio level thus effectively imporoving the apparent signal to noise ratio.

Most gear with headphone jacks generally have an attenutaor /resistor network installed between the audio amp of the rig and,the headphone jack for this very problem and,also to taken in to account more sensitive head phones.

73,

Bob.
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WB2WIK
Member

Posts: 20612




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« Reply #6 on: March 31, 2005, 02:47:06 PM »

General comment, although I don't have a TS-870:

I notice a lot of headphones nowadays are *very* sensitive and can be driven to earsplitting volume with about a 500 mW (1/2 W) source.  They produce intensely "loud" results even at 200 mW.

This is probably to make them compatible with the "Walkman" style battery-powered stuff which only produces about 100 mW of audio power and has to make that sound loud.

Since the TS-870 is a large, base-station rig and you really don't need to use sensitive headphones with it, you might simply try less sensitive headphones.  I do note if I use my lightweight SONY "Walkman" type headphones, even the big ones with large foam-cushioned ear cups, every piece of ham gear I own produces a loud "HISS" with the rig's volume turned all the way down.  Old-fashioned studio phones intended for use with big fat audio amplifiers don't do this.

WB2WIK/6
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N3ZKP
Member

Posts: 2008




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« Reply #7 on: March 31, 2005, 04:03:53 PM »

<< If you 'Pad down' the signal from the rig to the headphones and,turn up the volume control on the rig you effectively improve the signal to noise ratio. >>

While that is the result, you are still just dropping the signal level from the radio with a pad - nothing more, nothing less.

What you say about the source of the hiss is correct.

Lon
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2E0ESW
Member

Posts: 27




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« Reply #8 on: April 02, 2005, 12:17:13 AM »

My 2 cents worth,
e-mail Bob Heil, ask the man himself, he is very approachable and solved my similar probs on a Yaesu in one sentence. Worth a try and FREE.
73
Etienne Swanepoel
Bude
Cornwall
UK
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AA4PB
Member

Posts: 12899




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« Reply #9 on: April 02, 2005, 11:39:50 AM »

If you find that you are just barely opening the volume control to get sufficient audio level with the headphones then simply dropping the level to the headphones with a series resistor may indeed work. It will reduce the noise level in the headset and force you to add more signal by cranking up the volume control. If your volume control is already set to a "normal" level (perhaps 1/4 to 1/2) then you probably don't want to use this method.

It is also true that the "hiss" is high frequency noise and can be reduced with a parallel capacitor. The capacitor can reduce the level of the hiss without significanly reducing the level of the desired audio. If your volume control is already set to a normal range then you propably want to use the capacitor method. You may also find both a series resistor and a parallel capacitor (after the resistor) to be helpful.

The HS-5 is an 8-ohm headset designed for communications use. It has both a limited sensitivity and a limited frequency response. Since many communications receivers do not use very low noise output stages, the hiss is a common problem when trying to use a "hi fi" headset or speaker with these receivers. The HS-5 is my headset of choice for these reasons and because its open back permits me to still hear other activity in the room.
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KB2CPW
Member

Posts: 304




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« Reply #10 on: April 05, 2005, 08:16:59 AM »


  Funny,

  This is how the original Dolby Noise reduction worked.. By lowering the audio to a very low level and then reintroducing it to the amp stage, thus cutting the low frequency hiss.. A simple idea that made them millions.. Regards.. Richy N2ZD
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